Sampling of Indirect Dimensions In typical 2D correlation experiments the indirect dimension is sampled as a series of 1D measurements where a delay (sometimes called the evolution time) is incremented so that the effects of a desired interaction, such as J-coupling between protons in a COSY or the proton and carbon chemical shifts in an HSQC, can be observed. The example below (Figure 1) is from an HSQC data set sampled with 128 points. The size of the steps is the inverse of the spectral width. In this case, a time step of 0.05 ms corresponds to a sweep width of 20,000 Hz, or approximately 200 ppm at 100 MHz (the frequency of 13 C on a 400 MHz system). Similarly, the resolution is determined by the total time elapsed. In this particular case, 6.4 ms of total acquisition time (i.e., the longest time interval) corresponds to a resolution of 157 Hz in the indirect dimension. As an alternative, we can collect a smaller sample of the 1D spectra in the indirect dimension in order to speed up the experiment. This technique is known as Non Uniform Sampling (NUS). In this scheme, we collect only a portion of the points (in Figure 2 below, 25%) and fill in the missing 1D increments with zeroes.